The present invention relates to mechanical pumps and in particular to mechanical vacuum pumps incorporating at least one pair of intermeshing rotors, especially rotors of the type known as "claw" rotors.
When intermeshing claw type rotors are employed in mechanical vacuum pumps or compressors for use with gases or vapors which tend to condense or liquefy during the pumping or compression process, a hydraulic hammering effect is sometimes experienced. This hydraulic hammering effect is caused by the inability of the claw type rotors to expel liquid sufficiently rapidly from their swept volume The liquid thus accumulates to give the hammering effect which can lead to mechanical failure.
A typical claw type rotor mechanism is illustrated in FIGS. 1a to 1d of the accompanying drawings which shows a pair of rotors 5,8 mounted on respective shafts 1A, 1B for rotation about the shafts in the direction shown by the arrows with the claws 7,13 closely enain the walls of a chamber 10.
Any liquid formed in the volume swept by the rotors 5,8 or from vapor or entrained in gas entering through an inlet port 9 in the wall of the pumping chamber 10, tends to move radially outwards under centrifugal force towards the stator walls 2 away from the outlet port 3 which is located in the side wall 4 adjacent the centre of the rotor 5. The claws at their leading surfaces scoop the liquid as they rotate, and the claw 7 of the rotor 8 which is associated with the inlet port 9 throws the liquid towards the outlet port 3 as it rotates from the positions 1a through 1b to 1c of FIG. 1.
However, in the critical position between positions 1b and 1c, the outlet port 3 is closed and this prevents expulsion of the liquid from the pumping chamber 10. The liquid is thereby trapped between the rotors 5, 8 and creates an hydraulic hammering effect which can lead to mechanical failure of the pump.